This invention relates generally to lead-acid storage batteries, and more particularly to an electrolyte for such batteries which significantly improves the operating characteristics thereof.
Battery cells are referred to as "primary" batteries when they generate electrical current directly at the expense of chemical energy and can be renewed only by the replacement of the used-up materials. In a storage or "secondary " cell, the chemical changes which occur in furnishing current can be reversed by forcing current through the cell in the proper direction. In the charging process, chemicals which were used up in the ordinary running of the cell are manufactured anew. The most widely used secondary battery is the lead-acid cell; for its low cost, reliability and favorable performance characteristics render it acceptable for many different applications.
The lead-acid storage battery is manufactured in various sizes, ranging in capacity from less than one ampere-hour to several thousand ampere-hours. The most popular storage battery is the lead-acid automotive battery which is used for automobile starting, ignition and lighting, this battery having a moderate capacity, and a high-rate and low temperature performance.
In a lead-acid battery, use is made of a highly-reactive sponge lead for the negative electrode or cathode and lead dioxide as the active positive or anode electrode, a sulfuric acid solution serving as the electrolyte. As the battery discharges, the active material of both electrodes are converted into lead sulfate. Taking part in this reaction is the sulfuric acid electrolyte which, as the battery discharges, produces water. In the recharging process, the reverse action takes place. In the recharging process, the reverse action takes place. By measuring the specific gravity of the electrolyte, one can determine the state of charge, for the specific gravity decreases on discharge and increases on charge.
The discharge and charge reactions of the lead-acid cell are defined by the following expression: EQU Pb+Pb O.sub.2 +2H.sub.2 SO.sub.4 .revreaction.2 Pb SO.sub.4 +H.sub.2 O
in which the arrow to the right indicates the discharge reaction and that to the left the charge reaction. At the end of the charge, electrolysis of water also occurs, generating hydrogen at the cathode and oxygen at the anode.
The overall lead cathode reaction is given by the expression; EQU Pb+SO.sub.4.sup.= .fwdarw.Pb SO.sub.4 +2e.sup.-
The overall anode reaction is given by the expression: EQU Pb O.sub.2 +4H.sup.+ +SO.sub.4.sup.= .fwdarw.Pb SO.sub.4 +2H.sub.2 O-2e.sup.-
A pasted-plate design is commonly used in the construction of a lead-acid cell. The active material for each electrode is prepared as a paste by mixing divided lead oxides and suitable expander materials with sulfuric acid. The paste is spread onto a lead-alloy grid which affords the structure to hold the active materials as well as the necessary electrical conductivity. The resultant plates are then soldered to connecting straps to create negative and positive groups which are interleaved. To complete the assembly, separators are placed between the electrodes, the assembly then being housed within a container that is designed to include a sediment space under the assembly to collect any active material that is dislodged therefrom, as well as headroom above the assembly to hold excess electrolyte.
Conventional lead-acid batteries for automotive applications employ antimonial-lead grids to impart adequate strength to the thin grid structure and to facilitate casting. In recently-developed versions of such batteries, use is made of calcium-lead grids which are more resistant to corrosion and self-discharge. For applications other than automobiles, the lead-acid batteries are generally similar in design but differ in their lead-alloy composition, plate thickness, separators and containers in order to optimize the performance characteristics for the particular application.
Among the limitations of a lead-acid storage battery are its poor low-temperature characteristics and its loss of capacity on standing. It is well known that a lead-acid battery permitted to remain in a discharged state for more than six months will become "sulfated" and thereby difficult to discharge. Moreover, because of the relatively large amount of sulfuric acid included in conventional storage batteries, in the event of an accident resulting in spillage, this acid may become hazardous.